II-VI compounds have recently become of interest as possible materials for use in semiconductive diode injection lasers, particularly for operation in the blue-green portion of the optical spectrum. Such compounds can be prepared to have large band gaps suitable for emitting light in that portion of the spectrum. In particular, ZnSe has been used in the first blue-emitting diode lasers. However, such lasers tend to have low operating lives at room temperatures because of large heating effects.
Accordingly, there remains a need for longer lived lasers in the blue portion of the spectrum. Additionally, there is a need for laser operation at other portions of the wavelength spectrum. In particular, highly useful would be a materials system that could be used to tailor operation to chosen wavelengths over a relatively wide portion of the optical spectrum.
Generally, the operating range of an injection laser is determined by the band gap of the material in the layer of the laser where the injected holes and electrons recombine to emit the laser light. An injection laser generally comprises a multi-layered structure that includes a central layer, which is usually described as the active or recombination region where holes and electrons recombine to emit light and that is relatively free of dopants, and doped layers on opposite sides of the central layer, which are usually described as the cladding layers that serve as the source of the holes and the electrons, respectively, that are injected into the active layer for recombination. Additionally, the cladding layers are generally designed to have characteristic band gaps wider than those of the active region so that they serve as reflecting boundaries to help confine the emitted light within the active region. These cladding layers are also desirably of high conductivity both to keep power losses low and the heating effects small and to serve as good sources of the holes and electrons injected into the active layer.
There are a variety of considerations that go into the choice of the compositions of the various layers. First, the wavelength of the emitting light is determined by the band gap of the material forming the active layer so that the choice of this material is constrained by the output wavelength desired. Further constraints on this material are the need for the active layer to be lattice matched to the cladding layers so that the entire multilayer structure can be essentially monocrystalline with few dislocations or defects, a factor important for efficient operation.
There are various constraints also on the choice of materials for the cladding layers. As mentioned, they need to lattice match the active layer. Additionally, since it is usually necessary to grow the multilayered layer by epitaxial deposition over a suitable monocrystalline substrate the cladding layers need to be of a composition that will lattice match well with readily available substrates. Another constraint already mentioned is that the band gap of each of the materials forming the cladding layers needs to be wider than that of the active layer because of the confining role of the cladding layers. Another important constraint is that one of the cladding layers needs to be doped p-type and the other n-type so that a p-n junction can be formed at the active region. Moreover, if the heating losses in the laser are to be low, a factor important for long life, each of the cladding layers needs to be doped to concentrations high enough to achieve high conductivity, advantageously to concentrations of at least 10.sup.18 doping center per cubic centimeter
The doping problem is a common one in II-VI compounds, most of which can be doped relatively easily to be one conductivity type but resist efforts to dope them to the opposite conductivity type. Since most semiconductive devices depend for operation on the presence of two layer of opposite conductivity defining a p-n junction therebetween, II-VI compounds have found limited commercial use hitherto. In particular, because of the difficulties in doping posed by the II-VI compounds that have been studied extensively, there has been little recent activity in studying compounds such as MgSe and MgTe.